Effect of pyrocarbon interphase texture and thickness on tensile damage and fracture in T-700™ carbon fiber–reinforced silicon carbide minicomposites

In this paper, T-700™ carbon fiber–reinforced silicon carbide (C/SiC) minicomposites with pyrocarbon (PyC) interphase with different textural microstructure and thickness were fabricated using the chemical vapor infiltration method. The interface properties (i.e., textural microstructure, thickness, hardness, and modulus) were obtained through multiple testing methods (i.e., Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and nanoindentation tests). Relationships between the deposition temperature and residence time with the texture type (i.e., low, medium, and high texture) were established. Uniaxial tensile experiments were conducted for C/SiC minicomposites with different PyC interphases to characterize the composite's internal damage evolution and fracture. Relationships between the composite's tensile nonlinear damage evolution, fracture strength and strain, PyC interphase texture, and thickness were established. The composite's tensile strength and fracture strain were the highest for the C/SiC minicomposite with medium-high texture PyC interphase. For the C/SiC minicomposite with the same texture interphase, the composite's tensile strength and fracture strain were affected by the coating thickness. The higher the thickness of the coating, the lower the composite's tensile strength and fracture strain.     

Fabrication process and growth mechanism of novel micro-stack PyC interphase with different textures

Pyrolytic carbon (PyC) interphase plays a crucial role in the mechanical properties of fiber-reinforced ceramic matrix composites. In this research, a novel micro-stack PyC interphase with different PyC textures was designed and fabricated by changing the deposition parameters during the chemical vapor infiltration process. The growth mechanism of the micro-stack PyC interphase with different texture were also studied by experimental characterizations and kinetic calculations, and the results show that the content ratio of (C₂H₂ + C₂H₄) to C₆H₆ gas intermediate is a key parameter to control the texture types of PyC interphase. Furthermore, the value of orientation angle (OA) value, thickness, and modulus of the micro-stack PyC interphase were further characterized by high resolution TEM (HRTEM), scanning electronic microscopy, and nanoindentation. Finally, the tensile testing of mini-C_f/PyC/SiC composites was conducted, and the results showed that the tensile strength of mini-C_f/PyC/SiC composites with micro-stack PyC interphase is approximately 40% higher than that containing single high texture PyC interphase. The improvements on the tensile strength of C_f/PyC/SiC composites prove the significant advantages of micro-stack PyC interphase.     

High-temperature flexural strength of I-CVI processed Cf/SiC composites with variable interphases

The effect of single-layer pyrocarbon (PyC) and multilayered (PyC/SiC)_n=4 interphases on the flexural strength of un-coated and SiC seal-coated stitched 2D carbon fiber reinforced silicon carbide (C_f/SiC) composites was investigated. The composites were prepared by I-CVI process. Flexural strength of the composites was measured at 1200 °C in air atmosphere. It was observed that irrespective of the type of interphase, the seal coated samples showed a higher value of flexural strength as compared to the uncoated samples. The flexural strength of 470 ± 12 MPa was observed for the seal coated C_f/SiC composite samples with multilayered interphase. The seal coated samples with single layer PyC interphase showed flexural strength of 370 ± 20 MPa. The fractured surfaces of tested samples were analyzed in detail to study the fracture phenomena. Based on microstructure-property relations, a mechanism has been proposed for the increase of flexural properties of C_f/SiC composites having multilayered interphase.     

Mechanical properties and microstructures of Cf/SiC-ZrC composites using T700SC carbon fibers as reinforcements

C_f/SiC-ZrC composites were fabricated through mold-pressing and polymer infiltration and pyrolysis (PIP) process using T700SC carbon fibers as reinforcements. The effects of interphases on the mechanical properties and microstructures of composites were studied. Composite showed brittle fracture behavior and low bending stress of 81 ± 24 MPa when no interphase was deposited on the fiber surface. With the deposition of a PyC/SiC interphase, composite showed typical non-brittle fracture behavior and the bending stress increased to 401 ± 64 MPa and a large amount of pulled-out fibers could be observed on the fracture surface. Meanwhile, it could also be concluded from the microstructures of the composites that the existed interphases had a great hindering effect on the infiltration of ZrC into the intra-bundle zones in the slurry infiltration process. The TEM analysis results showed that the carbon fibers were almost not eroded and the brittle fracture behavior may be mainly ascribed to the strong bonding between fibers and matrix.     

Effect of interphase on mechanical properties and microstructures of 3D Cf/SiBCN composites at elevated temperatures

Interphase between the fibers and matrix plays a key role on the properties of fiber reinforced composites. In this work, the effect of interphase on mechanical properties and microstructures of 3D C_f/SiBCN composites at elevated temperatures was investigated. When PyC interphase is used, flexural strength and elastic modulus of the C_f/SiBCN composites decrease seriously at 1600°C (92 ± 15 MPa, 12 ± 2 GPa), compared with the properties at room temperature (371 ± 31 MPa, 31 ± 2 GPa). While, the flexural strength and elastic modulus of C_f/SiBCN composites with PyC/SiC multilayered interphase at 1600°C are as high as 330 ± 7 MPa and 30 ± 2 GPa, respectively, which are 97% and 73% of the values at room temperature (341 ± 20 MPa, 41 ± 2 GPa). To clarify the effect mechanism of the interphase on mechanical properties of the C_f/SiBCN composites at elevated temperature, interfacial bonding strength (IFBS) and microstructures of the composites were investigated in detail. It reveals that the PyC/SiC multilayered interphase can retard the SiBCN matrix degradation at elevated temperature, leading to the high strength retention of the composites at 1600°C.     

Mechanical properties and oxidation resistance of SiCf/CVI-SiC composites with PIP-SiC interphase

SiC coatings were successfully synthesized on SiC fibers by precursor infiltration and pyrolysis (PIP) method using polycarbosilane (PCS) as precursor. The morphology of as-fabricated coatings was observed by SEM, and its structure was characterized by XRD and Raman spectrum. The SiC fiber reinforced chemical vapor infiltration SiC (SiC_f/CVI-SiC) composites with PIP-SiC coatings as interphase were fabricated. And, the effects of PIP-SiC interphase on mechanical properties of composites were investigated. The experimental results point out that the coating is smooth and there is little bridging between fibers. The coating is amorphous with SiC and carbon micro crystals. The flexural strength of composites with and without PIP-SiC interphase is 220 and 100 MPa, respectively. And the composites with PIP-SiC interphase obviously exhibit a toughened fracture behavior. The oxidation resistance of composites with PIP-SiC interphase is much better than that of composites with pyrolytic carbon (PyC) interphase.     

Interphase degradation of three‐dimensional Cf/SiC–ZrC–ZrB2 composites fabricated via reactive melt infiltration

Layer‐structured interphase, existing between carbon fiber and ultrahigh‐temperature ceramics (UHTCs) matrix, is an indispensable component for carbon fiber reinforced UHTCs matrix composites (C_f/UHTCs). For C_f/UHTCs fabricated by reactive melt infiltration (RMI), the interphase inevitably suffers degradation due to the interaction with the reactive melt. Here, C_f/SiC–ZrC–ZrB₂ composite was fabricated by reactive infiltration of ZrSi₂ melt into sol‐gel prepared C_f/B₄C–C preform. (PyC–SiC)₂ interphase was deposited on the fiber to investigate the degradation mechanism under ZrSi₂ melt. It was revealed that the degraded interphase exhibited typical features of Zr aggregation and SiC residuals. Moreover, the Zr species diffused across the interphase and formed nanosized ZrC phase inside the fiber. A hybrid mechanisms of chemical reaction and physical melting were proposed to reveal the degradation mechanism according to characterization results and heat conduction calculations. Based on the degradation mechanism, a potential solution to mitigate interphase degradation is also put forward.     

In-situ tensile damage and fracture behavior of PIP SiC/SiC minicomposites at room temperature

In-situ tensile damage and fracture behavior of original SiC fiber bundles, processed and uncoated SiC fiber bundles, SiC fiber bundle with PyC interphase, SiC/SiC minicomposites without/with PyC interphase are analyzed. Relationships between load-displacement curves, stress-strain curves, and micro damage mechanisms are established. A micromechanical approach is developed to predict the stress-strain curves of SiC/SiC minicomposites for different damage stages. Experimental tensile stress-strain curves of two different SiC fiber reinforced SiC matrix without/with interphase are predicted. Evolution of composite’s tangent modulus, interface debonding fraction, and broken fiber fraction with increasing applied stress is analyzed. For the BX™ and Cansas-3303™ SiC/SiC minicomposite with interphase, the composite’s tangent modulus decreased with applied stress especially approaching tensile fracture; the interface debonding fraction increased with applied stress, and the composite’s tensile fracture occurred with partial interface debonding; and the broken fiber fraction increased with applied stress, and most of fiber’s failure occurred approaching final tensile fracture.     

Preparation and tensile behavior of SiCf/SiC mini composites containing different types and thicknesses of interphase

SiC fiber-reinforced SiC matrix (SiC_f/SiC) composites are promising materials for high-temperature structural applications. In this study, KD-II SiC fiber bundles with a C/Si ratio of approximately 1.25 and an oxygen amount of 2.53%, were used as reinforcement. PyC interphase, PyC-SiC co-deposition interphase I and II, with different thicknesses, and SiC matrix were deposited into the SiC fiber bundles by using chemical vapor infiltration (CVI) to form SiC_f/SiC mini composites. When the thickness of the interphase is approximately 1000 nm, the ultimate tensile stress and strain of SiC_f/SiC mini composites with PyC-SiC co-deposition interphase I can reach 1120.0 MPa and 0.72%, respectively, which are significantly higher than those of SiC_f/SiC mini composites with a PyC interphase (740.0 MPa, 0.87%) and PyC-SiC co-deposition interphase II (645.0 MPa, 0.54%). The effect of thicknesses and types of interphase on tensile fracture behavior of mini composites and then the fracture mechanism are discussed in detail.     

Appropriate thickness of pyrolytic carbon coating on SiC fiber reinforcement to secure reasonable quasi-ductility on NITE SiC/SiC composites

Pyrolytic carbon (PyC) coating of silicon carbide (SiC) fibers is an important technology that creates quasi-ductility to SiC/SiC composites. Nano-infiltration and transient eutectic-phase (NITE) process is appealing for the fabrication of SiC/SiC composites for use in high temperature system structures. However, the appropriate conditions for the PyC coating of the composites have not been sufficiently tested. In this research, SiC fibers, with several thick PyC coatings prepared using a chemical vapor infiltration continuous furnace, were used in the fabrication of NITE SiC/SiC composites. Three point bending tests of the composites revealed that the thickness of the PyC coating affected the quasi-ductility of the composites. The composites reinforced by 300?nm thick coated SiC fibers showed a brittle fracture behavior; the composites reinforced 500 and 1200?nm thick PyC coated SiC fibers exhibited a better quasi-ductility. Transmission electron microscope research revealed that the surface of the as-coated PyC coating on a SiC fiber was almost smooth, but the interface between the PyC coating and SiC matrix in a NITE SiC/SiC composite was very rough. The thickness of the PyC coating was considered to be reduced maximum 400?nm during the composite fabrication procedure. The interface was possibly damaged during the composite fabrication procedure, and therefore, the thickness of the PyC coating on the SiC fibers should be thicker than 500?nm to ensure quasi-ductility of the NITE SiC/SiC composites.     

The improvement of mechanical properties of SiC/SiC composites by in situ introducing vertically aligned carbon nanotubes on the PyC interface

In order to improve the mechanical properties, vertically aligned carbon nanotubes (VACNTs) were in situ introduced on the pyrocarbon (PyC) interfaces of the multilayer preform via chemical vapor deposition (CVD) process under tailored parameters. Chemical vapor infiltration (CVI) process was then employed to densify the multilayer preform to acquire SiC/SiC composites. The results show that the growth of VACNTs on PyC interface is highly dependent to the deposition temperature, time and constituent of gas during CVD process. The preferred orientation and high graphitization of VACNTs were obtained when temperature is 800?℃ and C₂H₄/H₂ ratio is 1:3. The bending strength and fracture toughness of SiC/SiC composites with PyC and PyC-VACNTs interfaces were compared. Compared to the SiC/SiC composite with PyC interface, the bending strength and fracture toughness increase 1.298 and 1.359 times, respectively after the introduction of PyC-VACNTs interface to the SiC/SiC composites. It is also demonstrated that the modification of PyC interface with VACNTs enhances the mechanical properties of SiC/SiC composites due to the occurrence of more fiber pull-outs, interfacial debonding, crack branching and deflection     

Transmission electron microscopy study of the microstructure of unidirectional C/C composites fabricated by catalytic chemical vapor infiltration

Unidirectional carbon/carbon (C/C) composites were fabricated by catalytic chemical vapor infiltration, using electroless Ni–P as catalyst. Transmission electron microscopy (TEM) investigations indicate that the catalyst particles (100–800 nm) in the pyrocarbon (PyC) matrix are composed of Ni₃P and Ni phases, but only the Ni₃P phase was observed in the tiny catalyst particles (<50 nm) in carbon fibers. The catalyst particles in the matrix were encapsulated by high-textured PyC shells, in which openings were observed. The thicknesses of the medium-textured PyC in the composites (720–850 nm) are greater than in conventional C/C composites (660–740 nm), but have no significant difference in texture degree. Catalysts were partially extruded out of the PyC shells and migrated into the carbon fibers, leading to the catalytic graphitization of the carbon fibers, and their structural homogeneity was destroyed. Based on the TEM observation, a dissolution/precipitation mechanism was proposed for the catalytic graphitization of carbon fibers, and a dissolution/precipitation/encapsulation/fracture/extrusion mechanism was proposed for the encapsulation of catalyst particles.     

Interfacial optimization of SiC nanocomposites reinforced by SiC nanowires with high volume fraction

High volume fraction SiC nanowires-reinforced SiC composites (SiCNWs/SiC) were prepared by hybrid process of chemical vapor infiltration and polymer impregnation/pyrolysis in this research. SiCNWs networks are first to be made promising a high volume fraction (20 vol%), and the pyrolytic carbon (PyC) interphase with 5 nm is designed on SiCNWs surface to optimize the bonding condition between SiCNWs and SiC matrix. Nanoindentation shows a modulus of 494 ± 14 GPa of SiCNWs/SiC composites without interphase comparing to the one with PyC interphase of 452 ± 13 GPa. However, the 3-point bending test shows a higher strength of the composite with PyC interphase (273 ± 32 MPa) comparing with the one without interphase (240 ± 38 MPa). The fracture surface is observed under SEM, which shows a longer SiCNWs pullout of the composite with PyC interphase. The energy dissipation during the 3-point bending test is calculated by the length of nanowire pull-out, it demonstrates that the SiCNWs with PyC interphase possess better performance for toughening composite. Further characterization proves that the PyC interphase can give SiCNWs/SiC composites higher fracture toughness (4.49 ± 0.44 MPa·m^1/2) than the composites without interphase (3.66 ± 0.28 MPa·m^1/2).     

Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) C_f/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the C_f/ZrC–SiC composites without and with a thin PyC interlayer of 0.15 µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2 µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2 µm. The highest flexural strength of 272.3±29.0 MPa and fracture toughness of 10.4±0.7 MPa m^1/2 were achieved for the composites with a 1.2 µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the C_f/ZrC–SiC composites slightly and the ZrO₂ scale acts as an anti-ablation component during the testing.     

Effects of SiC/HfC ratios on the ablation and mechanical properties of 3D Cf/HfC-SiC composites

Effects of SiC/HfC ratios on the ablation and mechanical properties of 3D C_f/HfC–SiC composites by precursor impregnation and pyrolysis (PIP) process were investigated systematically. Both strength (flexural and compressive strength) and modulus increase as the SiC/HfC ratio are improved. The compact and stiff HfC-SiC matrix in addition to the carbon fiber and PyC interphase with less reaction damage accounts for the improved mechanical properties of C_f/HfC-SiC with higher SiC/HfC ratios. Meanwhile, both weight loss and erosion depth of C_f/HfC-SiC are improved with the increased SiC/HfC ratios. Therefore, in order to balance the ablation and mechanical properties, an appropriate SiC/HfC ratio should be considered.     

Mechanical properties of SiCf/SiC composites with alternating PyC/BN multilayer interfaces

Alternating pyrolytic carbon/boron nitride (PyC/BN)_n multilayer coatings were applied to the KD–II silicon carbide (SiC) fibres by chemical vapour deposition technique to fabricate continuous SiC fibre-reinforced SiC matrix (SiC_f/SiC) composites with improved flexural strength and fracture toughness. Three-dimensional SiC_f/SiC composites with different interfaces were fabricated by polymer infiltration and pyrolysis process. The microstructure of the coating was characterised by scanning electron microscopy, X–photoelectron spectroscopy and transmission electron microscopy. The interfacial shear strength was determined by the single-fibre push-out test. Single-edge notched beam (SENB) test and three-point bending test were used to evaluate the influence of multilayer interfaces on the mechanical properties of SiC_f/SiC composites. The results indicated that the (PyC/BN)_n multilayer interface led to optimum flexural strength and fracture toughness of 566.0?MPa and 21.5?MPa?m^1/2, respectively, thus the fracture toughness of the composites was significantly improved.     

Large-scale synthesis of SiC/PyC core-shell structure nanowires via chemical liquid-vapor deposition

In carbon/carbon (C/C) composites, SiC/PyC core-shell structure nanowires were successfully fabricated via chemical liquid-vapor deposition (CLVD). The influences of heat-treatment temperature on the microstructure and composition of SiC nanowires were studied, and meanwhile the growth mechanism of SiC nanowires was discussed. Additionally, the microstructure and morphology of SiC/PyC core-shell structure nanowires were also investigated. The results displayed that the low heat-treatment temperature could not meet the requirements of SiC nanowires growth, but the too high temperature made the nanowires appear agglomerate easily. Only when the heat-treatment temperature was 1800 °C, SiC nanowires possessed a uniform distribution. The diameter of SiC nanowire was about 300 nm, and there was a SiO₂ layer with the thickness of about 1 nm existing on the surface of SiC nanowire. The growth behavior of SiC nanowire was governed by vapor-solid (V–S) mechanism. After the PyC deposition, SiC/PyC core-shell structure nanowires were constructed, and the nanowires were about 450 nm in diameter. These nanowires displayed a core-shell structure with three layers, which were SiC nanowire core, SiO₂ interlayer and PyC shell, respectively. Meanwhile, SiC/PyC core-shell structure nanowires connected the matrices with each other, and the core-shell structure nanowires generated a stable network.     

Achieving high permeability of in-situ formed lightweight Cf/SiC(rGO)px/SiC porous ceramics with improved interfacial compatibility for transpiration cooling

Making lightweight porous ceramics with excellent permeability applied for transpiration cooling is still challenging. Herein, an ingenious fabrication method is proposed to successfully prepare C_f/SiC(rGO)_px/SiC porous ceramics possessing low density, high permeability and satisfactory mechanical properties. The introduction of carbon fibers for constructing channels and SiC(rGO)_p with three-dimensional (3D) honeycomb cellular net-like structure, could effectively decrease density and improve porosity. Meanwhile, self-supporting porous skeleton, high open porosity and uniform pores distribution contribute to brilliant permeability of the products. Good interfacial compatibility among SiC(rGO)_p, carbon fibers and β-SiC/SiOxCy/C_free matrix, as well as toughening effects of carbon fibers are beneficial for enhancing fracture toughness and compressive strength. Particularly, C_f/SiC(rGO)_p0.6/SiC porous ceramics exhibit low density (1.12 g·cm^?3), low linear shrinkage (3.22%), especially high permeability (1.36 ×10^?7 mm²), satisfactory fracture toughness (1.77 MPa·m^1/2), excellent hardness (3.88 GPa) and compressive strength (6.41 MPa), focusing on potential applications as coolant medium in transpiration cooling.     

Tailoring Carbon Fiber/Carbon Nanotubes Interface to Optimize Mechanical Properties of Cf‐CNTs/SiC Composites

C_f/SiC composites were fabricated using fiber coatings including CNTs and matrix infiltration using the polymer impregnation and pyrolysis process. Interface between fiber and CNTs (CF/CNTs) was tailored to optimize mechanical properties of hybrid composites. The tailored interphases, such as Pyrocarbon (PyC) and PyC/SiC, protect fibers from degradation during the growth of CNTs successfully. Hybrid composites with well‐tailored CF/CNTs interface displayed significantly increased mechanical strength (352 ± 21 MPa) compared with that (34 ± 3 MPa) of composites reinforced with CNTs, which grown on carbon fibers directly. The interfacial bonding strength of hybrid composites was improved and optimized by tailoring the CF/CNTs interface. Interfacial failure modes were studied, and a firm interface bonding at the joint where CNTs grown was observed.     

Microstructure and mechanical properties of quasi-isotropic chopped fiber-reinforced PyC–SiC matrix composite prepared by novel nondamaging method

In this work, quasi-isotropic chopped carbon fiber-reinforced pyrolytic carbon and silicon carbide matrix (C_f/C–SiC) composites and chopped silicon carbide fiber-reinforced silicon carbide matrix (SiC_f/SiC) composites were prepared via novel nondamaging method, namely airlaid process combined with chemical vapor infiltration. Both composites exhibit random fiber distribution and homogeneous pore size. Young's modulus of highly textured pyrolytic carbon (PyC) matrix is 23.01 ± 1.43 GPa, and that of SiC matrix composed of columnar crystals is 305.8 ± 9.49 GPa in C_f/C–SiC composites. Tensile strength and interlaminar shear strength of C_f/C–SiC composites are 52.56 ± 4.81 and 98.16 ± 24.62 MPa, respectively, which are both higher than those of SiC_f/SiC composites because of appropriate interfacial shear strength and introduction of low-modulus and highly textured PyC matrix. Excellent mechanical properties of C_f/C–SiC composites, particularly regarding interlaminar shear strength, are due to their quasi-isotropic structure, interfacial debonding, interfacial sliding, and crack deflection. In addition to the occurrence of crack deflection at the fiber/matrix interface, crack deflection in C_f/C–SiC composites takes also place at the interface between PyC–SiC composite matrix and the interlamination of multilayered PyC matrix. Outstanding mechanical properties of as-prepared C_f/C–SiC composites render them potential candidates for application as thermal structure materials under complex stress conditions.